Optical fiber microwave systems offer an effective method for delivery of broadband wireless communication services. Optical fiber microwave systems are used in applications such as personal communications networks, millimeter-wave radio local area networks, broadband video distribution networks and signal distribution for phased array antennas. These systems could therefore play an important role in metropolitan area telecommunications networks. A key requirement of optical fiber microwave systems is an efficient technique for the optical generation of the microwave or millimeter-wave carrier signal.
Laser diodes have been looked at for this application, but they generally cannot be modulated at millimeter-wave frequencies. The use of external optical modulators has also been considered, since they can be modulated at millimeter-wave frequencies, but optical insertion loss is large and frequency-dependent. In addition, the direct modulation of a laser source and the amplitude modulation of an optical carrier signal by an external optical modulator result in the generation of dual side bands located on either side of the optical carrier and spaced from the optical carrier by the modulation frequency. As the optical signals propagate along an optical fiber the effects of chromatic dispersion result in a phase mismatch between the two sidebands, which leads, when a π phase mismatch occurs, to power nulls at the receiver. The effects of chromatic dispersion therefore limit the transmission distance to a short distance (approximately 4 kilometers for a 30 GHz modulation frequency on a 1550 nm optical carrier signal propagating in standard monomode fiber).
Another potential photonic carrier signal source consists of the heterodyning of the optical output signals from two lasers whose frequencies (wavelengths) differ by the required millimeter-wave frequency. This technique is currently favoured because it means that baseband (data) modulation need be applied to only one of the laser signals, and the limitations imposed by fiber dispersion on the signal transmission are greatly alleviated. Implementations of this technique include frequency locking, injection locking and the use of an optical phase locked-loop.
However, the heterodyning of the optical outputs from two separate lasers faces the problem that the phase noise on each of the laser signals is uncorrelated and this leads to the photonic carrier signal having an electrical linewidth which is greater than its optical linewidth. Even using an optical phase locked-loop will only stabilise the low frequency microwave phase variations in the photonic carrier signal as a result of the limiting delays in the feedback circuit.
Another potential photonic carrier signal source is the fiber laser since these generally offer bandwidths of the order of tens of kilohertz, potentially providing a photonic signal having a reasonably low frequency phase noise. In previously reported photonic signal sources using fiber grating-based lasers, the two optical frequencies originate from different optical cavities, resulting in high frequency phase noise on the generated photonic carrier signal. In another approach, using a multimode laser, the oscillating modes are generated in a single cavity. This results in each mode having a different amount of phase noise and competition between the two modes could destroy the heterodyning.